This invention relates to an apparatus and method for determining the penetration of the weld created from pulsed laser welding, and more particularly, to an apparatus and method of utilizing an optical technique to monitor the weld vaporization plume velocity to determine the depth of penetration.
Laser processing of metals is widely used in many industries, with the capability to monitor and control the depth of the weld into the metal crucial in many applications, such as nuclear safety critical welds.
In a laser welding process, the laser radiation first heats the metal surface. Melting occurs next, producing a plume of vaporized metal atoms and ions above the surface of the weld pool. When the laser energy input into the metal is sufficient, vaporization becomes sufficiently intense so that a void volume, or keyhole, is opened in the weld pool due to the recoil pressure generated by vaporization. During intense vaporization, a recoil pressure is developed due to the momentum change that occurs when metal atoms leave the surface of the weld pool. This recoil pressure is sufficient to cause a depression of the weld pool surface. As the laser intensity increases, this depression forms a void in the weld pool and allows for further penetration of the incident laser into the metal, leading to deep penetration welding. We refer to the regime of laser welding in which a void is formed and maintained in the weld pool during some fraction of the laser pulse as penetration, or keyhole, welding. In a pulsed laser welding process, the laser energy is delivered to the workpiece or part in short time intervals of approximately of 4-20 milliseconds. During each pulse, a vaporization plume is generated directly above the keyhole.
Several techniques, both non-optical and optical, have been disclosed for determining the state of penetration of a weld pool. Leong and Hunter, in U.S. Pat. No. 5,674,415 issued on Oct. 7, 1997, disclose a non-optical technique where an infrared signature emitted by a hot weld surface during welding is detected and the signature compared with a signature emitted during steady state conditions. This result in correlated with weld penetration.
Ortiz and Schneiter, in U.S. Pat. No. 5,045,669, issued on Sep. 3, 1991, disclose a laser apparatus that includes an optical sensor on the root side of the weld joint with a means for acoustically monitoring the processing. This technique requires accessibility to the root side of the joint.
Maram and Smith, in U.S. Pat. No. 4,767,911, issued on Aug. 30, 1988, disclose an apparatus that utilizes a photo-position detector to monitor the reflection angle of a beam of light specularly reflected from the weld pool to determine the state of penetration.